Direct Dyes: Direct dyes are also called substantive dyes because of their excellent substantivity for cellulosic textile materials like cotton and viscose rayon. This class of dyes derives its name from its property of having direct affinity for cellulosic fibres, when applied from an aqueous solution.


Properties of Direct Dyes: Chemically, direct dyes are sodium salts of aromatic sulphonic acids and most of them contain an azo group as the main chromophore. They are in general duller than the fibre reactive dyes, and exhibit poor wash fastness. Goods dyed with direct dyes unless; given a proper after treatment tend to bleed with every wash. The direct dyes in many cases exhibit a better light fastness as compared to the reactive dyes.


Classification of Direct Dyes: The direct dye in the colour index system refers to various planar, highly conjugated molecular structures containing one or more sulfonate groups which render solubility to the dye molecule. Their planar structure and length enable them to lie along side cellulose fibres and maximize the Van-der-waals, dipole and hydrogen bonds. As in other classes of dyes, the direct dyes can also be classified in many ways like the chemical nature of the chromophore, or by their dyeing characteristics and performance properties, but in general the most popular way of classification by the dyes is by means of their dyeing characteristics.


The classification of direct dyes by SDC is based essentially on the compatibility of different groups of direct dyes with one another under certain conditions of batch dyeing. There are three groups: A, B, and C.


Class A: Self Leveling Direct Dyes: These dyes have good leveling characteristics and are capable of dyeing uniformly even when the electrolyte is added at the beginning of the dyeing operation. Dyes in this group have good leveling properties and are capable of dyeing uniformly even when the electrolyte is added at the beginning of the dyeing operation.


Class B: Salt Controllable Direct Dyes: these dyes have poor leveling or migration properties. They can be controlled by addition of electrolyte, usually after the dye bath has reached the dyeing temperature. They require relatively large amount of salt to exhaust well.


Class C: Salt and Temperature Controllable Dyes. These dyes show relatively poor leveling or migration and their substantivity increases very rapidly with increasing temperature. Controlling rate of rise of the dye bath temperature, as well as controlling the salt addition, controls their rate of dyeing.


Besides, the dyeing behavior of different group of dyes, the dyeing process also operates at a wide range of liquor to good ratio (MLR), ranging from approximately 30-40:1, for loose raw stock in open vessels and for garment dyeing, approximately 20-30:1for becks; about 5-10:1 for jet dyeing machines; about 3-5:1 for package dyeing yarns and anywhere from about 0.6-2:1 for padding applications. As the Liquor ratios get lower, the solubility of the dye and the effect of the salt on the solubility get more critical. The solubilities of commercial direct dyes can range from about 5-300gms/litre at the boil. Dyes of high salt sensitivity, particularly at low liquor-to-goods ratios, can easily cause unlevel dyeings, because the strike rate is too high to allow for the dyeing uniformity, which comes from good circulation between the dye liquor and the fibre. In padding applications, differential strike rates in mixtures can lead to the initial preferential sorption of one or more of the components of the dye mix by the fabric. This leads to the phenomenon known as tailing or end-to end shading. The dyer especially in case of exhaust dyeing, consistently tries to optimize the liquor ratio, dyeing time and dyeing levelness, as a lower liquor ratio increases the chances of unlevel dyeing and requires a higher holding time to allow for the dye to migrate so that satisfactory dyeing in terms of shade build up and levelness is obtained.


Fibro has recently launched its latest series of process auxiliaries especially for the direct dyes, which consists of a leveling agent Fibrolev-DLV, and Dirofix-ECO an ecofriendly dye fixing agent. Both the products have been especially formulated for direct dyes, which are used for fabrics whose end-use do not involve regular washing.


 

Working Principle of Fibrolev-DLV: Cellulose fabric develops negative surface charge (zeta potential) when in contact with water in the dye bath. When direct dyes are introduced they being anionic in nature also posses a negative charge, resulting in repulsion between the like charges which prevents the dye from reaching the fabric surface (Figure 1a). The addition of electrolyte helps in neutralizing this negative zeta potential (Figure 1b) and hence facilitates the approach of the direct dyes towards the fabric. The direct dyes tend to rush towards the cellulose fabric surface after addition of an electrolyte (Figure 1c), or they may also agglomerate (Figure 1d) both these phenomenon result in patchy dyeing (Figure 1e). The addition of Fibrolev-DLV helps in level dyeing by forming a complex with the direct dye molecule, which disintegrates with rise in temperature. The dye molecules are hence released gradually and level dyeing is achieved (Figure 2a-2b).


The following experiment was carried out in our laboratory to evaluate the efficacy of these products.


Materials and Methods:


Fabric: 100% cotton canvas fabric.

Dyes Used: 1. Direct Fast Blue TLE 2. Direct Fast Yellow 5GLI 3. Direct Fast Turquoise Blue-GLH


Method:

Dyeing:


a. Dyerite Process: The fabric was initially treated with an wetting agent and Fibrolev-DLV, the temperature was then raised at 5.5C/min for 10 mins, dyes were then added in three installment followed by successive addition of electrolyte within 10 mins, with a rate of heating of 1.5C/min. The dyes were held at 95C/min for a period of 10 mins, and then the bath was drained.


Recipe:


Shade

= 3.0 %

Wetting agent

= 0.2 % Owf

Dirolev-Dlv

= 0.3 % Owf

Salt

= 15-20 g/l

MLR

= 1:10


b. Standard Dyeing Process: The normal dyeing was also carried out with a hold time of 1 hour at boil. Both the samples were then evaluated in terms of K/S values and their L, a, b values (A, B, C).


Recipe:


Shade

= 3.0 %

Wetting agent

= 0.2 % 0wf

Salt

= 15-20 g/l

MLR

= 1:10


After Treatment with Dye fixing Agent: The sample dyed with the aid of Fibrolev- DLV (A1,B1,C1) were then subjected to treatment with Dirofix-ECO. The samples were again evaluated in terms of their colour yield (K/S value), and L, a, b values.


Washing Fastness Test: These fabrics were then further subjected to washing fastness as per ISO 105/C01, the results of the same are indicated in Table 4.


Results and Discussion: Table 1 indicates the name of the different samples, with the treatment given.

The results as illustrated in Table 2 clearly indicate that the Dyerite process gives a similar colour yield (K/S value) as compared with the standard dyeing process. It can also be seen that there is no significant tonal differences which are indicated by the L, a, b, values.


The fabric were then subjected to after treatment with Dirofix-ECO and the comparison of the colour yield in terms of K/S value and tonal change by L, a, b, values are indicated in Table 3. The results in Table 3 do not show any adverse effect of Dirofix-Eco on the colour yield, and the tone of the shade.


The results hence indicate little or no colour change and with application of Dirofix-ECO. The visual examination of the wash liquor after application of the dyefixing agent, also indicated the absence of any colour in the liquid bath.


 

The washing fastness rating of the dyed fabric after treated with Dirofix-ECO as compared to that of the untreated sample were enhanced by 1 point as indicated in Table 4.


Table 1: Description of dyed fabrics subjected to various treatments.


Name of The Dye

Regular Dyeing Process

Dyerite Process

Dyerite Process+ Dirofix ECO

Direct Fast Yellow 5GLI

A

A1

A2

Direct Fast Turquoise Blue-GLH

B

B1

B2

Direct Fast Blue TLE

C

C1

C2


Table 2: Comparison of Colour Yield of Dyerite Process vs Regular Dyeing Process.


Sample Name

K/S

L

a

b

A

17.3757

72.474

10.672

78.920

B

10.6501

53.790

31.997

-25.564

C

11.8448

26.95

3.757

-19.596

A1

17.3555

71.728

10.875

78.780

B1

10.9201

53.908

31.871

-25.611

C1

11.9779

27.65

3.606

-19.348


Table 3: Effect of Dyefixing Agent on Colour Yield and Tonal Change of the Dyed Sample.


Sample Name

K/S

L

a

b

A1

17.3555

71.728

10.875

78.780

B1

10.9201

53.908

31.871

-25.611

C1

11.9779

27.65

3.606

-19.348

A2

17.2967

71.852

10.843

79.046

B2

10.6840

54.015

31. 754

-25.372

C2

11.9062

27.833

3.598

-18.888.


Table 3: Effect of Dyefixing Agent on Wash Fastness of the Dyed Sample.


Sample Name

Rating (staining on adjacent fabric)

A1

2

B1

2

C1

2

A2

3

B2

3

C2

3


Therefore, we can conclude that the application of Fibrolev-DLV, not only effects level dyeing, but also additionally helps in reducing the dyeing time. The combination of Direct dye leveling agent Fibrolev-DLV, and Dyefixing agent Dirofix-ECO gives excellent results, both in terms of colour yield and also performance properties with respect to wash fastness. The dyerite process increases productivity by reducing the dyeing time and also reduces correction and rejection processes by achieving level dyeing and reducing within and batch to batch tonal variations.





Figure 1a 1e: Dyeing of Cellulose with Direct Dyes in Absence of Leveling Agent.


 



Figure 2a 2b: Action of Dirolev-DLV on Direct Dyes.




Figure 3a: Flow-Chart of the Dyerite Process.


About the Authors:


The authors are associated with Fibro Organic India Pvt. Ltd.